1. Field of the Invention
The present invention relates to a light emitting device that uses a light emitting element and fluorescent material, and can be used for such applications as a signal light, illumination, display, indicator and other light sources, and particularly to a light emitting device that can emit light of white and other color tones by using fluorescent material that is excited by light emitted by a light emitting element, which emits in an ultraviolet to visible light region, and emits light in the visible light region.
2. Description of the Related Art
Various light emitting diodes (LED) and laser diodes (LD) have been developed as light emitting elements. These light emitting elements are replacing part of electric bulbs and cold cathode ray tubes that are used as light sources for display, back light and indicator, and the like by taking advantage of such features as a capability to run on low voltage, compact construction, light weight, small thickness, long life, high reliability and low power consumption. What deserves attention in this context are LEDs capable of emitting blue or green light with at least 10 candela luminous intensity that have been recently developed and commercialized by using an activation (light emission) layer of a quantum well structure made of a nitride semiconductor (for example, mixed crystal of InGaN). It has also been made possible to emit light of various colors including white light by combining light emitted by such LED chips and light emitted by fluorescent materials that are excited by light of the LED chips, thereby to produce fluorescence (principle of blending light colors).
An example of such devices is a light emitting device that uses a light emitting element which emits blue light and a YAG fluorescent material which is excited by the light emitted by the light emitting element and emits yellow light, so as to generate white light. This device emits white light by blending a transmitted part of the blue light from the light emitting element and yellow light emitted by the YAG fluorescent material that is excited by a part of the blue light from the light emitting element. This light emitting device has advantages of simple structure and ease of increasing output power.
There is also a device that emits white light by using a light emitting element, which emits an ultraviolet ray, and fluorescent materials capable of emitting light of RGB (red, green and blue) colors. There is also such a device that emits white light or other colors, wherein a fluorescent material which emits blue light when excited by a UV light emitting element and another fluorescent material which is excited by the blue light from the former fluorescent material emits yellow light (for example, refer to Patent Document 1). In this case, light color can be adjusted relatively easily since only the light emitted by the fluorescent materials is substantially used. When a light emitting element that emits an ultraviolet ray is used, in particular, mass productivity can be improved since color deviation due to wavelength of light emitted by this semiconductor light emitting element can be accommodated somewhat and chromaticity can be determined only by color of light emitted by the fluorescent materials, unlike a case of using a light emitting element that emits visible light.
[Patent Document 1]
Japanese Unexamined Patent Publication (Kokai) No. 2003-147351
However, there are demands for further improvement in a color rendering property for blended light emitted by a light emitting device that combines a blue light emitting element and YAG fluorescent material. There is a problem, in particular, that light emitted by such a device has an insufficient red component and a low value of a special color rendering index R9 that represents the red component. There is also such a problem that, since the device can emit light of only such colors that lie on a line connecting blue and a color of light emitted by YAG material on a chromaticity diagram, it is difficult to obtain light of a desired color by adjusting amounts of fluorescent materials; namely it is difficult to obtain a multi-color light emitting device.
A light emitting device that employs a light emitting element which emits an ultraviolet ray such as that described in Patent Document 1 has low intensity of emitted light at wavelengths around 500 nm, resulting in a low color rendering property.
It has also been attempted to improve a color rendering property by using at least two kinds of fluorescent material, such as combining a light emitting element which emits an ultraviolet ray and fluorescent materials capable of emitting light of RGB (red, green and blue) colors. But these attempts have been facing such a problem that difference in excitation characteristics among the fluorescent materials results in color deviation and/or variation in a color rendering property. For example, changing drive current density in the light emitting element causes a plurality of fluorescent materials to change light emitting characteristics differently, thus resulting in variations in color of emitted light.